Your Nose Smells in Stereo
Scientific understanding of olfactory information
content reaching the brain has just doubled
New experiments have shown that the brain can tell which nostril an odor came from, and that each nostril records a similar but different olfactory pattern. Combining these separate signals, the brain can tell where an odor came from and has double the information for analyzing a scent. It gets the information faster, too.
Each nostril has a unique sense of smell, intracranial electroencephalogram study finds (Medical Xpress, 6 Nov 2023). Experiments led by the University of Pennsylvania and the Barrow Institute of Neurology in Phoenix led to the discovery of stereo olfaction.
The study involved 10 subjects with intracranial depth electrodes in an odor identification task where odor stimuli were delivered to the left, right, or both nostrils through an olfactometer device designed to deliver odors by computer control.
Subjects had to identify the odor and indicate which nostril the odor came from. Subjects performed better in detecting and identifying odors in the bi-nostril condition compared to uni-nostril conditions, with no significant efficiency preference observed between the left and right nostril conditions.
An electroencephalogram recorded the responses. The nostrils are physically segregated by the nasal septum. Olfactory information is delivered to the piriform complex, the “smelling center” of the brain, which lies between the two hemispheres. The EEG showed similar but different neural activity to the uni-nostril tests, indicating that each olfactory bulb in the nose delivers a unique signal. The brain then integrates the information for higher resolution odor identification.
Coffee in stereo: your brain records an odour’s spatial information (Nature News, 3 Nov 2023). Reporter Saima Sidik says that scientists were unsure of how the brain responds to the two nostrils, whether separately or in unison.
To investigate this question, researchers recruited people with epilepsy who were undergoing brain surgery to identify the areas of their brains responsible for their seizures. Participants were awake for the surgery, during which the scientists delivered scents to one or both nostrils through tiny tubes that reached roughly one centimetre into each nostril. The authors took advantage of electrodes placed in the study participants’ brains to take readings of activity in the piriform cortex.
Naz Dikecligil, a neuroscientist at the University of Pennsylvania commented on the result, stating that “There seem to be actually two odour representations, corresponding to odour information coming from each nostril.”
When the researchers provided a scent to both nostrils simultaneously, they saw that both sides of the brain recognized the scent faster than either did when it was delivered through only one nostril. This suggests that the two sides do synergize to some degree, even though one lags behind the other in encoding a scent, Dikecligil says.
Multiple inputs improve sensation. We see in 3-D. We hear in stereo. Taste affects multiple sensors in the tongue. Touch comes from sensors all over the skin. And now we find that the sense of smell benefits from separate inputs.
The subjects were not accurate at telling the direction of a smell, but there is a benefit to having stereo inputs for odor identification.
When the researchers delivered a scent through the right or the left nostril and asked study participants which nostril a scent was coming from, they were no better at giving the correct answer than if they’d guessed randomly.
But it’s possible that dual signals provide an error-checking mechanism. Our brains might be “compiling corroborating evidence about what we’re smelling so we can accurately identify odour sources around us”, Bolding says.
The sense of smell is perhaps the most complicated of our senses, requiring rapid identification of thousands of odorants through multiple stages of encoding and decoding by olfactory receptor cells and downstream organs.
If stereo olfaction is at work in humans, it is probably active with even more precision in mammals with higher sensitivity to odors, such as dogs, mice, bears and elephants. It may even be a design feature in birds, fish, reptiles and insects. Moths, for instance, can fly direct toward extremely dilute concentrations of pheromones sensed from miles away.
Illustra Media’s film Living Waters illustrates how Pacific salmon can follow the scent of their natal stream for dozens or hundreds of miles, sensing concentrations measured in parts per billion.
Odor representations from the two nostrils are temporally segregated in human piriform cortex (Dikecligil et al., Current Biology, 3 Nov 2023). This is the research paper with the details.
One key feature that binds the olfactory systems of all mammals, including humans, is the presence of two nostrils, each conveying a potentially unique snapshot of the external olfactory world. Here, using iEEG recordings and machine-learning methods, we show that odor information from the two nostrils is temporally segregated in the primary olfactory cortex of humans. Furthermore, we demonstrate that greater differentiation among PC [piriform complex] odor representations scales with improved odor identification across subjects. Lastly, we show that the nostril-dependent order of odor representations influences when PC odor informs odor identification.
The authors found that, on average, odor identification using both nostrils was 480 milliseconds faster than with a single nostril alone.
It’s so nice to learn about new discoveries that expand our knowledge of the complexity of life. It’s especially nice when there is no mention of evolution. No Darwin story found in these articles, thankfully. We can make one up for evolutionists who feel deprived. Here’s one: “How the Shrew Got Its Nostrils.” Once upon a time, millions of years ago, mammals evolved two nostrils to improve the speed at which they could run from fires when smelling smoke.” Feel better now?